Plasmas and plasma generation has been studied for many years. There are several types of plasma generators currently employed for numerous applications. The well-known, atmospheric-pressure dielectric-barrier discharge is not a stable, continuous and homogeneous plasma; rather it is a series of short-lived, self-terminating arcs. This discharge generation system includes two parallel electrodes with a solid dielectric insulating layer on one of the electrodes. The dielectric layer serves to ensure termination of an arc. Substrates to be treated in such a discharge suffer local damage from the short-lived arcs (See, e.g., Y. Sawada et al., J. Phys. D: Appl. Phys. 28, 1661 (1995) and T. Yokoyama et al., J. Phys. D: Appl. Phys. 23, 1125 (1990)).
Microbeam plasma generators also require a dielectric material (quartz tube) between the inner and outer electrodes. Such microbeam devices operate at low power with high plasma gas flow velocities. The small deposition spot size created by the microbeam plasma generator can give rise to misleading deposition rates, and scale-up is likely difficult (See, e.g., H. Ha et al., J. Electrochem. Soc. 142, 2726 (1995), K. Inomata et al., Appl. Phys. Left. 64, 46 (1994), and H. Koinuma et al., Appl. Phys. Lett. 60, 816 (1992)).
Plasma torches are also referred to as thermal or arc discharges. These processes have been successfully utilized in the plasma spray metal coating industry. However, operating temperatures often exceed 10,000 Kelvin which is unacceptable in many situations (See, e.g., H. S. Uhm et al., Proceedings of the 1997 IEEE International Conference on Plasma Science, May 19-22, 1997, San Diego (IEEE, New York, 1997), p. 152, and A. Matsubara et al., Jap. J. Appl. Phys., Part. 1 35, 4541 (1996)).
Corona discharges have small exposed excitation area; thus, the quantity of generated reactive species is too small for industrial applications (See, e.g., E. Nasser, Fundamentals of gaseous Ionization and Plasma Electronics, Wiley-Interscience, New York, 1971, M. Goldman et al., Gaseous Electronics, Vol. 1, edited by M. N. Hirsh and H. J. Oskam (Academic Press, New York, 1978), p. 219-290, and R. S. Sigmond et al., Electrical Breakdown and Discharges in Gases, Part B, edited by E. E. Kunhardt and L. H. Luessen (Plenum Publishing Co., New York, 1983), p.1-64).
Low pressure plasmas are commonly used in the semiconductor industry for deposition, etching and ashing. This type of plasma requires a vacuum chamber with an expensive pumping system. In addition, the low pressure permits generated ions to impact the substrate which can damage underlying substrate layers and increase the substrate temperature (See, e.g., A. C. Adams et al., "Reduced Temperature Processing for VLSI," Electrochemical Society, Penington, N.J., 1986, F. S. Becker et al., J. Vac. Sci. Technol. B5, 1555 (1987), M. F. Ceiler, Jr., et al., J. Electrochem. Soc. 142, 2067 (1995), K. Ikeda et al., J. Electrochem. Soc. 143, 1715 (1996), K. Murase, Jap. J. Appl. Phys. 33, 1385 (1994), W. J. Patrick et al., J. Electrochem. Soc. 139, 2604 (1992), and S. K. Ray et al., Adv. Mater. For Optics and Electronics 6, 73 (1996)).
In U.S. Pat. No. 5,414,324 for "One Atmosphere, Uniform Glow Discharge Plasma," which issued to John R. Roth et al. on May 9, 1995, a one-atmosphere, steady-state glow discharge plasma is generated within the volume between a pair of insulated, equally spaced planar metal electrodes energized with an rms potential of 1-5 kV at 1-100 kHz is described. Roth et al. states that glow discharge plasmas are produced by free electrons which are energized by imposed direct current or rf electric fields. These electrons collide with neutral molecules transferring energy thereto, thereby forming a variety of active species which may include metastables, atomic species, free radicals, molecular fragments, monomers, electrons, and ions. An environmental isolation enclosure in which a low feed gas flow is maintained surrounds the plate assembly in order to equal the leakage rate of the enclosure. In fact, a no flow condition is taught for normal operation of the apparatus. Materials may be processed by passing them through the plasma between the electrodes, where they are exposed to all plasma constituents including ions. See, e.g., U.S. Pat. No. 5,403,453 for "Method And Apparatus For Glow Discharge Plasma Treatment Of Polymer Materials At Atmospheric Pressure," which issued to John R. Roth et al. on Apr. 4, 1995, and U.S. Pat. No. 5,456,972 for "Method And Apparatus For Glow Discharge Plasma Treatment Of Polymer Materials At Atmospheric Pressure," which issued to John R. Roth on Oct. 10, 1995.
Two patents by Hideomi Koinuma et al.: "Plasma Processing Method And Plasma Generating Device" which issued as U.S. Pat. No. 5,198,724 on Mar. 30, 1993 and "Plasma Generating Device" which issued as U.S. Pat. No. 5,369,336 on Nov. 29, 1994, describe a plasma generating device that includes a central electrode, a peripheral cylindrical electrode surrounding the central conductor, and an insulating cylinder interposed between the electrodes in order to prevent direct arc discharge from occurring therebetween. The electrodes and the insulating cylinder are coaxially arranged in order to define a cylindrical discharging space therein. By applying high-frequency electrical energy to the central electrode, a glow discharge is caused to occur between the central electrode and the insulating cylinder. A reactive gas is introduced from one end of the discharge space, excited by the glow discharge and exits from the other end as an excited plasma impinging on a work piece to be processed by the plasma. The Koinuma et al. apparatus cannot be scaled to large dimensions since the insulating cylinder must remain thin because it is required to conduct the radiofrequency discharge current. Further, the dielectric material is subject to attack by the reactive gases, and introduces a phase lag which requires that higher voltages and lower currents must be employed to maintain the discharge.
Accordingly, it is an object of the present invention to provide an apparatus for generating significant quantities of nonionic reactive species for materials processing over a large area outside of the plasma.
Another object of the invention is to provide an apparatus for generating significant quantities of nonionic reactive species for materials processing over a large area outside the plasma at atmospheric pressure.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.